1. Technical Field
The disclosure relates to a control apparatus for a power transmission system including an electrical transmission mechanism and a mechanical transmission mechanism in series with each other.
2. Description of Related Art
There is well known a control apparatus for a power transmission system including an electrical transmission mechanism, a second electric motor and a mechanical transmission mechanism. The electrical transmission mechanism includes a differential mechanism and a first electric motor. An engine is coupled to the differential mechanism such that power is transmitted. The first electric motor is coupled to the differential mechanism such that power is transmitted. A differential state of the differential mechanism is controlled by controlling an operating state of the first electric motor. The second electric motor is coupled to an output rotating member of the electrical transmission mechanism such that power is transmitted. The mechanical transmission mechanism constitutes part of a power transmission path between the output rotating member and drive wheels. This is, for example, a drive system for a vehicle, described in Japanese Patent Application Publication No. 2006-327583 (JP 2006-327583 A). JP 2006-327583 A describes that a known planetary gear-type automatic transmission is used as the mechanical transmission mechanism and the automatic transmission is shifted in accordance with a predetermined shift map.
Incidentally, when there is a malfunction that prevents the first electric motor from controlling the electrical transmission mechanism, the engine may be stopped by cutting off fuel. In such a case, in the electrical transmission mechanism, an engine rotation speed cannot be controlled by using the first electric motor, so the rotation speed of the first electric motor is determined on the basis of the rotation speed of the engine that is stopped in rotation and the rotation speed of an input rotating member of the mechanical transmission mechanism (which is synonymous with the rotation speed of the output rotating member of the electrical transmission mechanism) by consulting a relative relationship among the rotation speeds of rotating elements in the differential mechanism that performs differential action. The input rotating member of the mechanical transmission mechanism is determined on the basis of a vehicle speed and a gear ratio of the mechanical transmission mechanism. At this time, a high rotation speed of the input rotating member of the mechanical transmission mechanism may lead to an overspeed of the first electric motor. In contrast, in preparation for occurrence of the above-described malfunction, it is conceivable to cause the vehicle to travel while avoiding a traveling range, which is higher than or equal to the rotation speed of the input rotating member of the mechanical transmission mechanism and which leads to an overspeed of the first electric motor when the rotation of the engine is stopped, that is, to restrict use of a high vehicle speed and low gear ratio-side in the known shift map. However, the above restriction restricts acceleration of the vehicle up to a high vehicle speed at a low gear ratio even when a driver's acceleration request is large, so there is a possibility that drivability in traveling under normal conditions not in the event of malfunction is deteriorated.